1. Field of the Invention
The present invention relates to a photosensitive material, a coating dispersion including the photosensitive material, and an electrophotographic photoreceptor using the photosensitive material. In addition, the present invention relates to an electrophotographic image forming method, process cartridge and apparatus using the electrophotographic photoreceptor.
2. Discussion of the Background
Recently the growth of electrophotographic information processing apparatus (hereinafter image forming apparatus) such as copiers, printers and facsimiles is remarkable. In particular, photo-printers capable of recording digital information using light have been drastically improving in recording qualities and reliability. This digital recording technique is applied to copiers as well as photo-printers. The digital copiers to which this digital technique is applied have various image forming functions. Therefore it is considered that the demand for the digital copiers increases more and more.
At the present time, laser diodes (LDs) and light emitting diodes (LEDs) are used as light sources for the photo-printers because of being small in size, and having a relatively low cost and good reliability. The LEDs which are typically used for the photo-printers emit light having a wavelength of 660 nm. The LDs which are typically used for the photo-printers emit near infrared light. Therefore, a need exists for a photoreceptor having high sensitivities over a wavelength range including the visible region and the near infrared region.
The wavelength range over which an electrophotographic photoreceptor has sensitivity almost equal to the wavelength range over which the charge generation material used in the photoreceptor has photosensitivity. Therefore, various kinds of materials, such as azo type pigments, polycyclic quinone type pigments, trigonal system selenium, phthalocyanine pigments and the like, have been developed for the charge generation materials.
Among these pigments, titanyl phthalocyanine (hereinafter sometimes referred to as TiOPc), which has been disclosed in Japanese Laid-Open Patent Publications Nos. 3-35064, 3-35245, 3-37669, 3-269064 and 7-319179, is very useful for a photoreceptor for image forming apparatus such as printers and copiers, which use an LED or LD as a light source. This is because titanyl phthalocyanine is sensitive to light having a relatively long wavelength of from 600 to 800 nm.
A photoreceptor used for electrophotography such as Carlson process and the like processes is required to have the following charge properties as well as the high sensitivity to the specific light mentioned above:
(1) good charging ability such that a high electric potential can be formed and maintained in a dark place;
(2) good charge decaying ability such that the electric potential previously formed on the photoreceptor rapidly decays and the residual potential is low when the photoreceptor is exposed to light; and
(3) good charge stability such that the photoreceptor can maintain a good charging ability and a good charge decaying ability even when the photoreceptor is used for a long time.
In particulars in high sensitive photoreceptors such as photoreceptors including TiOPc, the charging ability thereof tends to deteriorate and the residual potential tends to increase when the photoreceptors are repeatedly used. Namely, the photoreceptors including TiOPc have an insufficient charge stability. Therefore, a need exists for a photoreceptor including TiOPc, which has good charge stability.
On the other hand, charging methods using a charging roller have been proposed to decrease the amount of ozone and NOx generated and to save charging energy. In the charging methods, the charging roller charges a photoreceptor while contacting the surface of the photoreceptor, or being close to the surface of the photoreceptor. It is certain that when a charging roller is used, the amount of ozone and NOx generated and the voltage applied to the charging roller can be decreased compared to the case using an indirect charging device such as scorotrons. However, a problem of breakdown of the photoreceptor tends to occur. The reason of the breakdown is not known yet. However, it is considered that the breakdown tends to occur because discharging is performed in a narrow gap between the photoreceptor and the charging roller. Effective solutions of this breakdown have not yet been proposed.
The methods for synthesizing TiOPc and the electrophotographic properties of TiOPc have been disclosed in Japanese Laid-Open Patent Publications Nos. 57-148745, 59-36254, 59-44054, 59-31965, etc. In addition, various crystal forms are known with respect to TiOPc, and Japanese Laid-Open Patent Publications Nos. 59-49544, 59-166959, 61-239248, 62-67094, 63-366, 63-116158, 63-196067, 64-17066 etc. have disclosed TiOPcs having a different crystal form.
The TiOPc crystal disclosed in Japanese Laid-Open Patent Publication No. 61-239248 has good electroconductivity, and therefore, the photoreceptor which includes only the TiOPc crystal as the charge generation material has good photosensitivity. However, the photoreceptor has a large charge decay in a dark place. In addition, the charging ability of the photoreceptor deteriorates when the photoreceptor is repeatedly used.
Such problems occur even when the TiOPc crystal having an X-ray diffraction spectrum such that a diffraction peak is observed at a Bragg (2θ) angle of 7.5° is used together with another TiOPc crystal, for example, having a peak at an angle of 27.2°.
When a photoreceptor is improved so that image defects mentioned above are not produced, the life of the photoreceptor mainly depends on the abrasion amount of the photosensitive layer thereof. In a conventional charge transport layer in which a low-molecular-weight charge transport material in a molecular state are dispersed in an inactive polymer, it is needed to increase the concentration of the charge transport material to increase mobility of the charge transport material. When the concentration of the charge transport material increases, the abrasion resistance of the charge transport layer deteriorates.
To improve the abrasion resistance, a charge transport polymer, in which a group having a charge transport function is incorporated in a polymer, is used in the charge transport layer. The photoreceptor including a charge transport polymer has relatively good abrasion resistance compared to the photoreceptor including a low-molecular-weight charge transport material. In addition, a group having a charge transport function can be included in a polymer at a high concentration, and therefore a charge transport layer having high mobility can be prepared. However, charge transport polymers generally have a relatively large gas permeability compared to the photoreceptor including a low-molecular-weight charge transport material because the group having a charge transport function is typically bulky. Therefore, the photoreceptor including a charge transport polymer has poor resistance to gases. Therefore, problems caused by ozone and the like gases tend to occur when the photoreceptor is used for a long time.
Because of these reasons, a need exists for a photoreceptor having high sensitivity, good charge stability and good resistance to gases.